Influence of upper airway pressure changes on control of breathing will be studied. Special emphasis will be given to the respiratory timing and to the peak activity of diaphragm and upper airway muscles such as genioglossus. In anesthetized, tracheostomized, spontaneously breathing rabbits, the upper airway will be isolated into a closed system so that pressure changes can be made at will. Diaphragm and genioglossus muscle electromyograms will be recorded with bipolar fine electrodes. In the first two studies, using a servo respirator triggered by the genioglossus muscle activity, negative and positive pressure pulses (magnitude: 5 or 10 cm. water; duration 200 msec) will be applied to the isolated upper airway either early or late in inspiration or expiration. Alteration in peak muscle activity as well as the alterations in respiratory duration (inspiratory and expiratory durations) of the experimental breaths will be compared to their respective control values. In the third study, the role of upper airway mechanoreceptors in the genioglossus response to neck flexion will be evaluated. The peak genioglossus muscle activity during neck flexion when the animal is breathing throught the nose will be compared to tracheostomy breathing. Similarly, the increase in genioglossus activity during neck flexion will be compared to similar values obtained after bilateral superior laryngeal nerve section in some animals and after midcervical vagotomy in others. In the fourth protocol, the effect of various levels of hypoxia and hypercarbia on the reflex response to upper airway pressure changes will be determined. These influences (large cyclic negative pressure, neck flexion, hypoxia and hypercarbia) are usually present in obstructive apnea. Such apnea are seen in a variety of clinical states: adults and children with sleep apnea, premature infants and, possibly, in SIDS (sudden infant death syndrome). The proposed projects are anticipated to provide valuable insight into the maintenance of upper airway patency in such conditions.